Radial turbines, which are configured to rotate a turbine rotor by introducing exhaust gas from a spiral scroll part formed inside a turbine housing radially toward blades of the turbine rotor located inside the scroll part to apply a force on the blades, then discharging the gas in an axial direction, have been commonly employed in turbochargers (exhaust gas turbochargers) of internal combustion engines or the like of cars.
FIG. 6 shows an example of a conventional turbocharger that uses such a radial turbine disclosed in Japanese Patent Application Laid-open No. 2003-120303 (Patent Document 1). Referring to the drawing, reference numeral 01 denotes a turbine housing, 04 denotes a spiral scroll part formed inside the turbine housing 01, 05 denotes an exhaust gas discharge passage formed by the inner circumference of the turbine housing 01, 06 denotes a compressor housing, and 09 denotes a bearing housing that connects the turbine housing 01 and the compressor housing 06.
Reference numeral 010 denotes a turbine wheel that has a plurality of circumferentially equally spaced turbine blades 03 fixed around the rim. Reference numeral 07 denotes a compressor impeller, 08 denotes a diffuser provided at the air outlet of the compressor impeller 07, and 012 denotes a rotor shaft that connects the turbine wheel 010 and the compressor impeller 07. Reference numeral 011 denotes a pair of bearings attached to the bearing housing 09 to support the rotor shaft 012. Reference symbol L1 denotes the rotation axis of the turbine wheel 010, the compressor impeller, and the rotor shaft 012.
In the turbocharger with such a radial turbine, exhaust gas from an internal combustion engine (not shown) enters the scroll part 04, and while traveling around along the spiral shape of the scroll part 04, it flows to the turbine blades 03 from the outer circumferential inlet ends thereof, then radially flows toward the center of the turbine wheel 010. Having done the expansion work on the turbine wheel 010, the gas then flows in the direction of axis line L1 of the rotor shaft 012 and is discharged from the exhaust gas discharge passage 05 to the outside of the turbocharger.
FIG. 7A shows a schematic configuration diagram of a cross section, in a direction perpendicular to the axis line L1 of the rotor shaft 012, of a tongue portion and its vicinity formed inside at the exhaust gas inlet of the radial turbine of Patent Document 1. FIG. 7B is a diagram viewed from a direction of arrow W in FIG. 7A.
In FIG. 7A, 04 denotes the scroll part, 044 denotes the exhaust gas inlet, and 045 denotes a tongue portion, which is formed at a connecting part between a flow passage 046, which the exhaust gas from the exhaust gas inlet 044 passes through to be introduced into the scroll part 04, and a blade side passage 047, into which the gas flows toward the blades, the tongue portion 045 separating the two passages 046 and 047.
The tongue portion 045 is subjected to the heat of exhaust gas both from the flow passage 046 and the blade side passage 047 as shown in FIG. 7B. Moreover, the tongue portion 045 has poor heat dissipation efficiency to dissipate the accumulated heat because of the narrow heat dissipation path as indicated by the arrow Z.
Accordingly, the temperature of the tongue portion 045 can sometimes reach 800 to 900° C.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-120303
In such a radial turbine, the high-temperature exhaust gas from the engine passes through the flow passage 046, and while traveling around along the spiral shape of the scroll part 04, it flows out into the blade side passage 047.
As the tongue portion 045 is exposed to the high-temperature exhaust gas from both of the flow passage 046 and the blade side passage 047, and because of its heat dissipation path being only in the direction Z (see FIG. 7A), heat tends to accumulate. The tongue portion 045 is therefore prone to surface oxidation due to the high temperature, or to fatigue damage resulting from thermal stress.
A countermeasure for this problem is to use a material having good oxidation resistance and fatigue resistance at high temperatures (such as cast austenitic stainless steel or cast ferritic stainless steel) for the turbine housing 01, which is expensive and causing an increase in cost.